The talk-radio host, a favorite of conservatives, confirms that he is under investigation and tells his fans that he is seeking drug treatment.

"In terms of radio, Rush has been the impetus of several things that have happened," said Ken Mueller, former radio curator for the Museum of Television & Radio. "One is sort of the rebirth of AM radio. There are probably a lot of radio station owners -- especially in small markets -- if not for the likes of Rush Limbaugh, they'd be off the air."

In a statement, former U.S. Secretary of Health, Education and Welfare Joseph A. Califano Jr. praised Limbaugh's decision to enter treatment.

"Rush Limbaugh is one of hundreds of thousands of Americans that are addicted to pain medication," Califano, president of the National Center on Addiction and Substance Abuse at Columbia University, said. "He has taken an important first step in admitting that addiction and seeking treatment. I wish him well."

Limbaugh has faced health issues before. In October 2001, he announced over the air that he was going deaf. The broadcaster and many of his fans feared that he would be unable to continue his radio program.

Physicians at the House Ear Institute in Los Angeles determined the cause was a rare autoimmune inner-ear disease, akin to arthritis. Limbaugh's immune system was attacking his body and creating inflammation that robbed him of his ability to hear.